I've found a new way to search the GRO indices if you know a mother's maiden name. For common surnames like Elliott it is painstakingly tedious, but after an hour I have identified all 4 grandchildren:
Ivo's children: 1 dau (born 2014) and 1 son (Tristan Hugh L ELLIOTT, born 2016 above)
Nico's children: 2 sons (born 2014/2016 respectively, the eldest named above)